1. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to a method and apparatus for monitoring fiber tension during the drawing of such fibers.
2. Description of the Prior Art
As is well known in the art, a critical parameter in the production of optical waveguide fibers is the tension within the fiber during the drawing process, and, in particular, the tension within the fiber in the region between the hot zone and the first coater. The magnitude of this tension affects the final properties of the fiber, including the fiber's diameter, ultimate strength and, through a stress optic effect, its optical properties.
From a process point of view, the tension in the fiber also affects the overall stability and throughput of the drawing process. Excessive tension leads to rapid necking and ultimate rupture of the fiber in the hot zone region. If not carefully controlled, increasing the temperature of the hot zone to reduce fiber tension can result in draw resonance and root oscillation, which in turn can result in oscillatory variations in fiber diameter which are difficult to control with conventional fiber diameter monitoring equipment. Oscillation of the fiber during drawing can also adversely affect the fiber coating process.
To date, fiber tension has been monitored mechanically by measuring the deformation of the fiber in response to a force applied transversely to the direction of motion of the fiber. Specifically, a three wheel device has been used wherein two wheels are applied to one side of the fiber and a third wheel is applied to the other side of the fiber. The third wheel is lightly sprung and can move in and out relative to the first two wheels. The location of the third wheel relative to the first two wheels has been used as a measure of the tension in the fiber.
The three wheel approach has numerous disadvantages which arise from the fact that the approach involves physically contacting the moving fiber. One such disadvantage results from the fact that it is difficult to precisely align the device with the fiber so as not to change the original path of the fiber. Even slight changes in fiber path can have a significant effect on the overall drawing process. For example, as described below, it has been found that contact of the three wheel device with the fiber will affect the on-line fiber diameter feedback loop so as to reduce fiber draw speed. Probably the most serious disadvantage of physically contacting the moving fiber, however, is breakage of the fiber. Because of this problem, the three wheel approach has generally only been used for monitoring fiber tension at draw speeds below about 5 meters/second.